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author: Oldrich Pecak
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# Sequential circuits II, Memory

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## time aware XOR

- time-exclusive XOR
- works like XOR, but also looks 1 cycle into the past
- let's use JK flip flops

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## Memory

- useful (obviously)
- can be built using flip flops
    - depends on type of memory
- many types
    - ROM
    - PROM
    - EPROM
    - EEPROM
    - RAM
    - Registers
- can be synchronous or asynchronous

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## Implementing logic using memory

- combinational logic -> LUTs
    - look up tables are common in FPGAs
- sequential circuit
    - only requires some additional logic around
    - can be programmable

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## Let's program a state machine

- let's revisit the two circuits we build before

1. Pulse length counter
2. Time aware XOR

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## Homework

- build a 4x4 bit memory cell in Digital
- use only flip flops and logic
- the memory has to have the following commands:
    - clear all
    - write
    - read
- input/output data bus can be separate